Theoretical Study of Phase Transition Behaviour in Mixed Biopolymer + Surfactant Interfacial Layers Using the Self-Consistent-Field Approach - Food Colloids: Self-Assembly and Material Science - page 253

16.3.2 Density Profiles
In adsorbed layers involving macromolecules, the interfacial region can be
rather thick. The variation of the density profile of the biopolymers in this
interfacial region can provide useful information on possible configurations
adopted by the adsorbed chains. 22 In particular, changes in the structure of the
mixed films, at the transition point, can be obtained by studying the concen-
tration profiles of the biopolymers, for both the low- and the high-coverage
surfactant phases. Using the SCF calculations, we have determined the variation
of the concentration profile of the protein precisely at the transition points; this
is plotted as a function of distance away from the solid surface in Figure 5. The
profile for the high-coverage surfactant phase is represented by the solid line and
that for the low-coverage surfactant phase by the dashed line. Although the free
energies associated with each profile are exactly identical at the transition point,
the variations in the biopolymer concentration profiles, with the distance away
from the solid surface, are manifestly different. For the low-coverage surfactant
phase, a high number of predominantly hydrophobic polymer segments are in
contact with the wall, with the hydrophilic parts of the chains dangling into the
solution. For the high-coverage surfactant phase, on the other hand, the number
of contacts between the chains and the solid surface is much reduced. In the
latter phase, even the hydrophobic sections of these molecules form extended
loops protruding into the bulk solution. This gives rise to a distinct peak in the
shape of the profile, clearly visible at a distance of around five monomer units
away from the surface. The interfacial layer, not surprisingly, is also found to be
somewhat more extended for the high-coverage surfactant phase.
In Figure 6, we have plotted the average distance away from the surface,
o R i 4, for each polymer segment i. The segments are numbered consecutively

0.25

0.20

0.15

0.10

0.05

0.00

0 5 10 15 20 25 30

distance from the solid surface (a o )

Figure 5 Variation in the biopolymer density profile for the high-coverage (solid line) and
low-coverage (dashed line) surfactant phases, plotted as a function of distance
(in monomer unit size a 0 ) from the solid interface. The results are obtained at
the transition point